Hank Green: The fall equinox is just a few days away, marking the end of summer and the beginning of autumn, and everybody remembers what the equinox is because of its name -- "equinox", Latin for "equal night". It's the time of year, it comes once in March and once in September, in which night and day are the same length everywhere in the world.

Lies, friends, all lies.

I know! That's what the name says that it is, and that's what most of us were taught in schools, but science, guys! Science is always making sure that stuff is more interesting than it sounds.

[intro music]

For starters, the equinox isn't even a day, it's a moment -- a specific moment of astronomical alignment, when then Earth's axis is exactly perpendicular to the Sun, neither pointing toward it or away from it. This alignment occurs when the geometric center of the Sun is directly above the equator.

Now, on these days it is indeed true that the center of the Sun appears above the horizon for 12 hours, but for several reasons, the day is actually longer than the night.

The first reason is that the Sun is just freakin' huge, so at sunrise the disk of the Sun appears above the horizon several minutes before the center does, and likewise, even after the center drops below the horizon in the evening, it's not officially sunset until the top edge of the disk disappears.

Secondly, we observe the Sun through our beautiful, breathable atmosphere which refracts the Sun's rays, so we can actually see the top edge of the Sun before it even rises above the horizon in the morning, and in the evening we can see it for a few minutes after it has actually set. This causes every day, including the days of the equinoxes, to appear at least six minutes longer than it physically is, and also helps cause some awesome sunsets, so you can thank it for that too.

And both of these effects are actually amplified the farther away you get from the equator. If you're sitting with your butt on the equator watching the path of the Sun on equinox day, you'd see that both at sunrise and sunset it would intersect with the horizon at exactly 90°, but at higher latitudes the angle of the Sun's approach to the horizon gets smaller, and the Sun takes a lot longer to rise and set at a shallow angle than when it's going straight up and down.

So you put these effects together, and it starts to add up. For example, in Quito, Ecuador, just below the equator the length of the equinox day is about 12 hours and 6.5 minutes. In Shanghai, at 30° north latitude, it's 12 hours and 8 minutes, and in St. Petersburg, Russia, at 60° north latitude it's 12 hours and 16 minutes. Not a huge difference, but still, the nox is not quite equi. Stupid Latin!

Also, stupid astronomers, always making up Latin terms that aren't quite accurate. This makes me so angry!

Thanks for watching this episode of SciShow. If you know of any other things, terms, ideas, scientific concepts that are poorly understood, please let us know so that we can explain them to the world! If you have questions, ideas, suggestions, or comments, you can leave them on Facebook or Twitter or of course in the comments below, and we'll see you next time.

[outro music]